Manufacturers of EMI shielded cabinetry, such as computer cabinets, need to provide a clean conductive surface on the support frames and enclosure panels onto which the EMI shielding may be attached. This has required a bare or plated surface be formed in these areas. A plated surface is preferred as it provides areas of high conductivity and corrosion resistance. The remainder of the cabinet is generally painted with an electrostatic discharge coating or a finish coat of paint. The plated areas need to be masked during the painting sequence. The mask is then stripped away, the EMI shielding is applied and the cabinet is assembled.
While this method has proven to be acceptable to the cabinet industry, the cost of plating such cabinets has increased dramatically due in part to the environmental considerations involved in such plating processes as well as the scarcity of platers and the cost of transporting the parts to and from the plater.
An alternative to the plating of the cabinet surfaces consists of a conductive metal foil having one side coated with a pressure sensitive conductive adhesive and the other side covered with a peelable plastic film such as a Mylar.RTM. plastic film. The product is known as CHO-MASK.RTM. and is available from Chomerics, Inc. The masking tape is applied to the desired areas of the cabinet which is then painted or provided with a finish coat. The cover film is then removed to reveal a conductive, noncorrosive surface to which the EMI shielding gasket is mated. The mask had one of two different peelable surfaces depending upon whether the finish coat on the cabinet was subjected to a bake cycle. If so, the peelable coat is designed to release upon being heated.
While generally acceptable, this mask system has several disadvantages. Customer confusion as to which type of peel system to be used often led to the use of the improper mask which resulted in the failure of the mask to either withstand the bake cycle or peel at room temperature. Additionally, the peel system used in the bake cycle applications has to be removed while still hot due to the adhesive system that is used. This type of mask is difficult if not impossible to remove when at room temperature and often results in rips or tears in the conductive foil which reduces the EMI shielding capability of the system in those locations. Further, an adhesive residue which remains on the foil surface after the cover layer must be removed. This residue can effect the shielding capabilities of the foil and thus required the additional step of cleaning the foil surface before applying the EMI shielding. Perhaps most importantly, as the cover layer is removed, it tends to chip or nick the paint along the edges of the foil, leaving exposed unplated or areas of the cabinet which can be subject to corrosion. Further, these nicks or chips allow air to get under the conductive foil which can subject the conductive adhesive layer to oxidation and subsequent failure.